A thermal inkjet method has conventionally been known as an information output apparatus used for a word-processor, personal computer, facsimile apparatus, and the like.
Especially according to a thermal inkjet method, many nozzles can be formed on a printhead at high density.
Also, such a thermal inkjet type printhead (to be referred to as a printhead hereinafter) adopts a printing element (to be referred to as a heater board hereinafter) in which a heater for heating ink, a protective film for the heater, a driver circuit for supplying a current to the heater, a logical circuit for controlling the driver circuit, and the like are integrated into a single-crystal silicon substrate by a semiconductor integrated circuit manufacturing process.
FIG. 9 is a circuit diagram showing the circuit configuration of a heater and its driving circuit inside a conventionally known heater board.
In FIG. 9, reference numeral 101 denotes each heater for heating ink; 102, each driver transistor serving as a switch for supplying a current to the heater; 103 and 104, wiring resistances which are parasitic in wiring lines for connecting the heater 101 and driver transistor 102 to pads for electrical connection to the outside of the printhead; and 105 and 106, pad terminals VH (positive potential) and GNDH (reference potential) for externally applying power, respectively.
As is apparent from FIG. 9, the heater 101 and driver transistor 102 are in one-to-one correspondence. By selecting and driving any desired driver transistor, supply of a current to a corresponding heater can be controlled. At this time, the driver transistor is selected and driven by supplying an element selection output from the internal circuit (not shown) of the heater board to a gate terminal 107 of the driver transistor.
As shown in FIG. 9, an electrical connection from the pad to the heater and driver transistor is achieved by an independent wiring line for a group of adjacent heaters and driver transistors. With this wiring, variations in resistance value depending on the distance between each heater, each driver transistor, and the pad fall within a predetermined range. In addition, a voltage drop by the wiring resistance is kept constant in each group by limiting the number of simultaneously energized heaters in each group to one.
Many heaters formed on the heater board are desired to ideally generate uniform powers by all heaters without any variations. In practice, however, powers vary.
The variations in power are caused by variations in the resistances of wiring lines which connect heaters and pads serving as electric contacts on the heater board, variations in the ON resistances of driver transistors for controlling energization to heaters, variations in the resistance values of heaters, and the like.
Variations in power generated by the heater are directly influenced by variations in not the resistance value of the heater but the wiring resistance and the ON resistance of the driver transistor. That is, as a resistance other than the heater resistance increases, the increased resistance consumes the voltage and current, and power generated by the heater decreases.
Influence by variations in heater resistance is reduced against power variations.
More specifically, when each heater resistance becomes higher than a design center value owing to variations in heater resistance, a voltage applied to the heater rises, but a flowing current decreases. Since generated power is expressed as the product of the voltage and current, the increase in voltage and the decrease in current are canceled. As a result, the influence becomes smaller than the influence of variations in resistance other than the heater resistance.
FIG. 10 is a circuit diagram schematically showing a circuit configuration for supplying a current to one heater.
In FIG. 10, Rh represents the resistance of the heater 101; and Rp, the sum of a wiring resistance other than the heater resistance, and a parasitic resistance 108 such as the ON resistance of the transistor. These resistances are series-connected.
Assume that, as design center values, the heater resistance is Rh=100 Ω, the parasitic resistance including the wiring resistance and ON resistance is Rp =50 Ω, and the power supply voltage (V) between terminals is V=15 V.
Since the total resistance of the system is 100 Ω+50 Ω=150 Ω and the voltage between terminals is 15 V, the current (I) isI=V/R=15 V/150 Ω=0.1 AUnder these conditions, power generated by the heater is given byP=V×I=R×I2=100 Ω×(0.1 A)2=1 W
Assuming that the parasitic resistance varies from the center value 50 Ω to 60 Ω, power generated by the heater changes as follows.
More specifically, since the total resistance of the system is 100 Ω+60 Ω=160 Ω and the voltage between terminals is 15 V, the current (I) isI=V/R=15 V/160 Ω=0.0938 AHence, the generated power isP=V×I=R×I2=100 Ω×(0.0938 A)2=0.880 W
Assuming that the resistance value of the heater varies from the center value 100 Ω to 110 Ω while the parasitic resistance remains at the center value of 50 Ω, power generated by the heater changes as follows.
More specifically, since the total resistance of the system is 110 Ω+50 Ω=160 Ω and the voltage between terminals is 15 V, similar to the above example, the current (I) isI=V/R=15 V/160 Ω=0.0938 AHence, power generated by the heater is.P=V×I=R×I2=110 Ω×(0.0938 A)2=0.968 W
From the above example, the influence of variations in heater resistance to a larger value is smaller on variations in power than on variations in parasitic resistance. Similarly, the influence of variations in heater resistance to a smaller value is smaller on variations in power than on variations in parasitic resistance.
As described above, the resistance of a heater serving as a printing element formed on a heater board, and parasitic resistances such as the wiring resistance and the ON resistance of a driving transistor vary in resistance value.
Whether or not variations in resistance values fall within the tolerance is determined by electrically measuring the resistance values. The measurement has conventionally been done by measuring the total resistance of the heater resistance and parasitic resistance.
As is apparent from the above description, variations in power generated by the heater change depending on whether the cause of variations in resistance value is the heater resistance or parasitic resistance (see Japanese Patent Publication Laid Open No. 2003-11373).
From the viewpoint of stable operation check of the printhead, necessary information is variations in power generated by the heater. The variations have conventionally been measured by measuring the total resistance of the heater resistance and parasitic resistance. It is, therefore, difficult to identify whether the cause of variations is the heater resistance or parasitic resistance. It is obvious from the above consideration that when the cause of variations is the heater resistance, the tolerance can be made wider than that for the parasitic resistance.
However, the prior art must always prepare for the worst case because it is difficult to identify the cause. The specifications must be defined so that no problem occurs even when all measured variations are caused by the parasitic resistance. If power generated by the heater satisfies the specifications of the product, but the total resistance value of the system does not satisfy the specifications, the heater board is determined as a defective. This decreases the yield.